An aromatic ether polymer has been widely studied as a gas separation membrane due to the high physical strength, high solvent resistance, and high water resistance which is expressed based on its high hydrophobic property and particularly due to the very excellent gas permeability in the past.
For example, in the case of assuming a gas separation membrane for separating nitrogen gas and oxygen gas, since it is difficult to separate nitrogen gas from the gas composition mixed with oxygen gas by the difference in their molecular sizes, it is usually necessary to cause the membrane to express its separating performance by the difference in solubility and diffusibility of each gas component in the membrane. Therefore, in order to increase the gas separation and permeation performance, major effort has been directed to densifying the pore size on the membrane surface to 1.0 nm or less (refer to Non-patent Document 1), providing a dense layer as thin as possible on the membrane surface (refer to Patent Document 1), and increasing the gas solubility rate into the membrane material by suitable chemical modification, for example, by introducing an appropriate charge structure onto the membrane surface.
From the viewpoint of the membrane manufacturing process and the membrane structure, a hollow fiber membrane produced by a melt casting/spinning method utilizing thermally-induced phase separation has been reported (refer to Non-patent Document 2). The resulting hollow fiber membrane has a uniform porous structure from the inner surface to near the outer surface of the hollow fiber, and only the outermost surface has a completely blocked non-porous structure. The membrane is useful as a membrane aimed for gas separation.
A dense membrane of an aromatic ether polymer having a surface chemically modified by providing a charge structure, for example, has been used for a number of other applications, in which it is difficult to separate the target substance by the difference in the molecular size, by utilizing the difference in dissolution and/or diffusion in the membrane. Such a membrane is used, for example, as a reverse osmosis membrane for removing or concentrating inorganic salts having a size of an atomic or ionic level or as a pervaporation membrane for separation of low molecular compounds having a molecular size very close to each other. In designing such a membrane, efforts have also been directed to increasing the performance by densifying the surface pore size to 1.0 nm or less, making the membrane non-porous, and controlling the charge strength, and the like. For example, it has been reported that a sulfonated aromatic ether polymer membrane (refer to Non-patent Document 3) and an aminated aromatic ether polymer membrane (refer to Non-patent Document 4) exhibit excellent ion separating effects. However, since the sulfonated aromatic ether polymer membrane shows strong anionic properties and the aminated aromatic ether polymer membrane shows cationic properties during a liquid treatment using water as solvent, problems such as adsorption and denaturation occur depending on the materials to be treated. Moreover, the aromatic ether polymer must be chemically modified in order to introduce a charge structure and the like. This may cause problems such as reduction of physical strength, a decrease in solvent resistance, and the like.
When a membrane provided for the treatment of liquid, which is as important as the gas separation membrane, is considered, size of the material to be separated is generally 2.0 nm or more (refer to Non-patent Document 1). The major mechanism of separation that is applied is thus size separation. Therefore, in order to separate various materials to be separated in a liquid depending on their size, it is necessary to manufacture a porous membrane of which the pore size is precisely controlled based on the size of the materials to be separated.
An aromatic polysulfone polymer is given as an excellent material used for a membrane for treating liquid. The aromatic polysulfone polymer has many good solvents which are miscible with water at about room temperature and it is possible to produce various membranes such as a flat membrane, a hollow fiber membrane and the like by using a wet-phase separation membrane formation (refer to Non-patent Document 1 and Patent Document 2). Moreover, since the aromatic polysulfone polymer resin can be molded by melt molding, it is also possible to produce various membranes by a melt casting/spinning method. That is, the aromatic polysulfone polymers have large flexibility of membrane manufacturing, and membranes with a required pore size can be freely manufactured.
On the other hand, an aromatic ether polymer has scarcely been used as a membrane for treating liquid. The melting point of the aromatic ether polymer is very close to the thermal decomposition temperature in comparison with an aromatic polysulfone polymer. Thus, it has been difficult to use the aromatic ether polymer for manufacturing a membrane by a melt casting/spinning method in which sufficient melting is indispensable while inhibiting decomposition of resin. In addition, an aromatic ether polymer having high solvent resistance, differing from the aromatic polysulfone polymer in solubility to organic solvents, does not have a good solvent miscible with water, which is commonly used as a coagulating liquid, at around room temperature. Ultrafilter membranes made from an aromatic ether polymer which have been reported heretofore were manufactured by a method of using a casting/spinning solution of which a good solvent is a halogen-containing non-aqueous organic solvent (refer to Non-patent Document 5). Such a method has been difficult to carry out industrially due to a heavy environmental load and high production cost. Manufacture of membrane from an aromatic ether polymer by a wet-phase separation membrane formation using a halogen-free aqueous organic solvent at around room temperature has also been very difficult. Furthermore, it is very difficult to obtain a stable and homogeneous casting/spinning solution by blending a hydrophilic polymer as a hydrophilizing agent to the casting/spinning solution for the wet-phase separation membrane formation containing aromatic ether polymer, because the solubility of the aromatic ether polymer is decreased.
In respect of performance, the high hydrophobic property of the aromatic ether polymer is a problem when the aromatic ether polymer is used as a membrane for using water or a water-soluble organic solvent such as an aqueous solution or an alcohol as a filtering solvent or when the aromatic ether polymer is used for separating proteins.    [Patent Document 1] JP-A-03-65227    [Patent Document 2] Japanese Patent No. 2782583    [Non-patent Document 1] Membrane Technology (2nd edition), IPC, edited by Marcel Mulder, supervised and translated by Masakazu Yoshikawa, Tsuyoshi Matsuura, and Tsutomu Nakagawa (1997), pp. 16, 45-51, 256, 275-280    [Non-patent Document 2] Journal of Membrane Science Vol. 116, pp. 171-189, 1996: S. Berghmans, J. Mewis, H. Berghmans, H. Meijer    [Non-patent Document 3] Desalination, Vol. 155, pp. 229-242, 2003: Isabelle M. Noel, Remi E, Lebrun, Christian R. Bouchard    [Non-patent Document 4] Journal of Membrane Science Vol. 215, pp. 25-32, 2003: Xu Tongwen, Yang Weihua    [Non-patent Document 5] Desalination, Vol. 22, pp. 205-219, 1977: L. Broens, D. M. Koenhen, C. A. Smolders